Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene is likely to increase, due to a developing shortage of propylene. Thus, a process that uses higher alkenes instead of propylene as feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenols.
For example, oxidation of cyclohexylbenzene (analogous to cumene oxidation) could offer an alternative route for phenol production without the problem of acetone co-production. This alternative route co-produces cyclohexanone, which has a growing market and is used as an industrial solvent, as an activator in oxidation reactions, and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam and nylon 6. However, this alternative route requires the development of a commercially-viable process for producing the cyclohexylbenzene precursor.
It has been known for many years that cyclohexylbenzene can be produced from benzene by the process of hydroalkylation or reductive alkylation. In this process, benzene is heated with hydrogen in the presence of a catalyst such that the benzene undergoes partial hydrogenation to produce cyclohexene which then alkylates the benzene starting material. Thus U.S. Pat. Nos. 4,094,918 and 4,177,165 disclose hydroalkylation of aromatic hydrocarbons over catalysts which comprise nickel- and rare earth-treated zeolites and a palladium promoter. Similarly, U.S. Pat. Nos. 4,122,125 and 4,206,082 disclose the use of ruthenium and nickel compounds supported on rare earth-treated zeolites as aromatic hydroalkylation catalysts. The zeolites employed in these prior art processes are zeolites X and Y. In addition, U.S. Pat. No. 5,053,571 proposes the use of ruthenium and nickel supported on zeolite beta as the aromatic hydroalkylation catalyst. However, these earlier proposals for the hydroalkylation of benzene suffer from the problems that the selectivity to cyclohexylbenzene is low, particularly at economically viable benzene conversion rates, and that large quantities of unwanted by-products, particularly cyclohexane and methylcyclopentane, are produced.
More recently, U.S. Pat. No. 6,037,513 has disclosed that cyclohexylbenzene selectivity in the hydroalkylation of benzene can be improved by contacting the benzene and hydrogen with a bifunctional catalyst comprising at least one hydrogenation metal and a molecular sieve of the MCM-22 family. The hydrogenation metal is preferably selected from palladium, ruthenium, nickel, cobalt and mixtures thereof and the contacting step is conducted at a temperature of from about 50 to 350° C., a pressure of from about 100 to 7000 kPa, a benzene to hydrogen molar ratio of from about 0.01 to 100 and a WHSV of from about 0.01 to 100. The '513 patent discloses that the resultant cyclohexylbenzene can then be oxidized to the corresponding hydroperoxide and the peroxide decomposed to the desired phenol and cyclohexanone.
According to the present invention, it has now been found that in the hydroalkylation of benzene over a bifunctional catalyst comprising an MCM-22 family zeolite and a hydrogenation metal, the selectivity to cyclohexylbenzene, and it dialkylated counterpart, dicyclohexylbenzene, is highly dependent on the conversion rate of benzene feedstock. The selectivity of cyclohexylbenzene is the mathematical quotient of the hydroalkylation conversion rate (i.e., benzene conversion rate when benzene is the limiting reactant) divided by the cyclohexylbenzene yield. In particular, it is found that the selectivity of cyclohexylbenzene is optimized when the hydroalkylation conversion rate is especially in the range of from about 15% to about 75%. In this range of hydroalkylation conversion, the cyclohexylbenzene selectivity is optimized and ranges of from about 45% to about 85%, while the selectivity to dicyclohexylbenzene ranges of from about 5% to about 40%.